One of the biggest problems in measurement of sewage sludge or any other flowable substance is a dirt build-up on the sensor elements which are in contact with the flowable substance. The effect of the layered dirt on the microwave sensor elements may cause an error of up to tens of percentages to a measurement result. Such a huge error may lead to wrong control of a process, excessive or imbalanced use of chemical agents or other input material and also economic losses. Thus, there is a need for improvement in the microwave measurement of flowable substance.